1. Field of the Invention
This invention relates to an OFDM radio communications system conducting radio communications in the OFDM (Orthogonal Frequency Division Multiplexing) scheme.
2. Description of the Related Art
According to known macro-diversity receiving technology, a required gain can be obtained by transmitting the same pilot signal and data signal with the same carrier frequency from a plurality of transmitters and synthesizing the signals by a receiver. “MBMS transmission in E-UTRA”, from LG Electronics, discloses a frame configuration for conducting macro-diversity in the OFDM radio communications system wherein signals to which macro-diversity reception is applied and signals to which macro-diversity reception is not applied are present together.
In the frame configuration disclosed in “MBMS transmission in E-UTRA”, pilot signals alone are scrambled by a scrambling pattern defined for each of the transmitters. Data signals are not scrambled. The scrambling pattern is set such that the pilot signals have an orthogonal or pseudo-orthogonal relationship with one another. In this frame configuration, the pilot signals can be separated from signals transmitted from other transmitters on the receiver side by being orthogonalized or pseudo-orthogonalized with the signals from other transmitters. Therefore, the pilot signals can be used for estimation of a channel response to the signals to which the macro-diversity reception is not applied.
To conduct the macro-diversity reception for the signals transmitted in the frame configuration disclosed in “MBMS transmission in E-UTRA”, two-stage processings need to be executed on the receiver side. At the first stage, the responses of channels from the transmitters to the receiver are estimated individually by taking advantage of the matter that the scrambling pattern applied to the pilot signals has an orthogonal or pseudo-orthogonal relationship in the transmitters. At the second stage, channel equalization, i.e. equalization of compensating for channel distortion which the received data signals undergo is conducted by utilizing the sum of the individually estimated channel responses. Thus, the data signals transmitted to the receiver from the transmitters over the respective channels can be subjected to channel equalization.
The channel responses from the transmitters to the receiver need to be estimated individually, to conduct the macro-diversity reception on the receiver side for the signals of the frame configuration disclosed in “MBMS transmission in E-UTRA”. In other words, although the sum of the channel responses alone is essentially required for the channel equalization in the macro-diversity reception, the channel responses need to be obtained individually in “MBMS transmission in E-UTRA”. For this reason, the calculation amount is increased.
On the other hand, in the processing of estimating the channel responses individually, the channel response of the desired channel alone is taken out by canceling the channel responses of the other channels by taking advantage of the matter that the scrambling pattern has an orthogonal or pseudo-orthogonal relationship in the transmitters. However, the orthogonality or pseudo-orthogonality is restricted due to the channel distortion and the effect of canceling the other channel responses may be made smaller. In this case, since the other channel responses are superposed on the desired channel response as interference, the estimation accuracy of the desired channel response is deteriorated.
Furthermore, in the processing of estimating the channel responses individually, the receiver needs to recognizes all the scrambling patterns included in the receive signal. If the receiver does not recognize some of the scrambling patterns, the receiving performance is deteriorated since the channel responses from the transmitters to the receiver, corresponding to the respective scrambling patterns cannot be estimated. In addition, if the receiver identifies by mistake the scrambling pattern which is not included in the receive signal, an estimation of the channel response generated by the misidentified scrambling pattern includes only interference and the receiving performance is thereby deteriorated. To identify the scrambling patterns, the receiver needs to control the information on to which scrambling pattern the received signal corresponds and, therefore, the control on the receiver side becomes complicated.
Thus, in a case where the transmitted signal is subjected to the macro-diversity reception on the receiver side in the conventional configuration disclosed in “MBMS transmission in E-UTRA”, the problems such as an increase of the calculation amount, deterioration of the channel estimation accuracy caused by the interference, and complicated controls are caused.